Fix short flag parsing to strip equals sign from value

[Arpi223]

When parsing short flags with equals syntax (e.g., -o=value), the parser was incorrectly including the equals sign in the parsed value, resulting in '=value' instead of 'value'. This was inconsistent with long flag parsing (--option=value) which correctly returned 'value'.

This fix strips the leading equals sign when present, making short and long flag parsing consistent.

This fixes issues in tools like FlutterFire CLI where users expect -i=bundleid to work the same as --ios-bundle-id=bundleid.

Fixes #908

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[davidmorgan]

As commented on the issue--I think this is working as intended.

[natebosch]

I agree, we should keep the current and documented behavior.

If we were starting today I would argue against the current behavior and for restricting abbreviated arguments to flags instead of options. The most common behavior I'm familiar with is that a single dash can be followed by one or more single character flags. ls -la passes both the -l and -a flags.

I don't think it's worth breaking existing usage, but if it were I would drop the ability to abbreviate options rather than tweak the way the parsing for abbreviated options works.